Circulating water systems are commonly used to supply the cooling demands in process plants in which cooling water is an acceptable medium for heat exchange.
After passage of the circulating water through the heat exchange equipment, the water typically is cooled by passing through a cooling tower. An evaporative cooling tower commonly provides the heat removal necessary to return the circulating water to its supply temperature.
The heat transfer process in the cooling tower involves (1) latent heat transfer owing to vaporization of a small portion of the water and (2) sensible heat transfer owing to the difference in temperature of water and air. The major heat transfer is due to latent heat with a small percent due to sensible heat.
Due to the evaporation which takes place during cooling, the concentration of the dissolved solids in the circulating water increases. The water lost by evaporation must be replaced by makeup water.
The concentration of dissolved solids in the circulating water becomes greater than in the makeup water due to this evaporation loss. The term "cycles of concentration" is used to indicate the degree of concentration of the dissolved solids in the circulating water as compared with the makeup. For example, 2.0 cycles of concentration indicates that the concentration of dissolved solids in the circulating water is twice that of the makeup water.
While the evaporation loss tends to cause the dissolved solids in the water to concentrate, the windage or drift loss, which is the loss of fine droplets of water entrained by the circulating air, tends to limit the degree of concentration. This occurs because dissolved solids are present in the droplets of water and thereby leave the system with the drift loss. The drift loss, although not as large an amount as the evaporation loss, also represents a loss of water from the system and this likewise must be replaced by makeup water.
In circulating water systems, where the dissolved solids in the water are concentrated by evaporation, the problem of scale formation is increasingly troublesome. Scale is formed by solids in the water which have decreasing solubilities in water with increasing temperature. Common scale forming solids are calcium carbonate, calcium sulfate, calcium silicate and magnesium silicate. Scale formation results when the concentrations of scale forming solids exceed saturation concentrations, and the dissolved solids precipitate out of solution. Water that possesses scale forming tendencies, that is, water that has high concentrations of compounds that can form scale, becomes even more scale forming when concentrated. Even water that is not scale forming, in which, for example, the initial concentrations of these compounds is low, usually becomes scale forming when concentrated two, four or six cycles.
If solids are allowed to build up in a circulating water system, scale formation can become a serious problem. Scale can deposit and grow on the walls and surfaces of heat exchange equipment. These deposits can negatively affect heat transfer performance and restrict cooling water flow through the equipment. Eventually, it becomes necessary to shut down the process plant to clean the scale from the heat exchange equipment. The cost of cleaning and the losses associated with process plant operation down time can be extremely high.
The primary scale problem in circulating water systems is calcium carbonate scale formation, although it is also necessary to take steps to prevent the deposition of calcium and magnesium silicate and calcium sulfate scale. The low solubility of calcium carbonate, especially at higher temperatures, make it the primary target for scale reduction.
Known calcium carbonate scale treatment methods include the addition to the makeup water of anti-nucleating agents. such as polyphosphate tannin, as well as organic and inorganic surface active agents. These agents cause crystal distortion and are effective in decreasing scaling tendencies in recirculating conditions.
Relatively high concentrations of these agents are required, however, to achieve the nonscaling effect. Up to 100 ppm of these agents is required in the circulating water.
The solubility of calcium sulfate is further extended in circulating systems through the use of dispersants, sequestrants or chelants. Dispersants prevent buildup of particle size through adsorption, which involves electrostatic forces. Sequestrants and chelants function through electron transfer, forming a water soluble complex with calcium. These agents would also typically be added to the makeup water entering the circulating system and can require high treatment concentrations.
Increasing treatment concentrations cannot always control the scale problem, however. To avoid oversaturation in the circulating water with respect to calcium carbonate, as well as calcium and magnesium silicate, it is necessary to limit the cycles of concentration by means of blowdown. Blowdown consists in removing a portion of the circulating water. The concentrated water removed as blowdown is replaced with additional fresh makeup water, thus lowering the concentration in the system. Blowdown can be either intermittent or continuous. The rate of blowdown is varied to maintain the cycles of concentration within safe limits to prevent scale formation.
While blowdown is an effective method for limiting cycles of concentration and hence the scaling potential of the circulating water, it requires excessive rates of makeup water. In many locales, the supply of fresh water is either limited or costly. It would be desirable to have treatment measures that permit higher cycles of concentration in the circulating water, with corresponding lower frequencies of blowdown and required quantities of makeup water.
Known methods used to reduce blowdown while lowering scale formation include makeup stream softening and acid treatment. Softening of makeup water is most often accomplished through ion exchange.
"Hydrogen zeolite" is the name given to a group of non-siliceous organic materials, either natural or synthetic, which are capable of exchanging hydrogen ions for cations such as calcium, magnesium and sodium. See, e.g., Betz Handbook of Industrial Water Conditioning, Sixth Edition, 1962, Chapter 15, pp. 96-101. When the makeup water stream which contains calcium, magnesium and/or sodium ions is passed through a hydrogen zeolite these ions are exchanged for hydrogen, and bicarbonate, sulfate, nitrate and chloride radicals are converted to their respective acids: carbonic acid (H.sub.2 CO.sub.3), sulfuric acid (H.sub.2 SO.sub.4), nitric acid (HNO.sub.3) and hydrochloric acid (HCI).
When the hydrogen zeolite bed becomes exhausted it is backwashed and regenerated with acid. It is then reused. Large investment cost is associated with such softening equipment. There is also a considerable operating cost associated with the high acid demand required to regenerate the hydrogen zeolite bed following treatment of the makeup water stream.
The carbonic acid generated in the hydrogen cation exchange decomposes to gaseous carbon dioxide and water in the treated makeup water stream. A decarbonator is commonly used to mechanically remove the carbon dioxide thus formed. The remaining acids, however, must be neutralized.
Neutralization of the acid stream is achieved by adding a suitable alkali, such as caustic soda (NaOH) or soda ash (Na.sub.2 CO.sub.3) to the treated makeup water stream. This can also be a significant cost associated with present treatment methods, especially when the total volume of water is large and the total concentration of ions in the makeup water is high.
Acid treatment is another known method for reducing scale formation in a circulating water system. In conventional acid treatment, acid is added to the water in the circulating water system typically by mixing the acid with the makeup water or by adding the acid directly to the cooling water basin. Sulfuric acid is generally used because of its lower cost.
By treating the water with sulfuric acid, calcium bicarbonate in the water is converted to the more stable and more soluble calcium sulfate, as illustrated by the following reaction: EQU Ca(HCO.sub.3).sub.2 +H.sub.2 SO.sub.4 .fwdarw.CaSO.sub.4 +2CO.sub.2 +2H.sub.2 O
Thus the degree of calcium carbonate oversaturation can be lowered through sulfuric acid treatment.
There are problems associated with conventional acid treatment, however. Even the more soluble calcium sulfate is capable of forming scale in high concentrations. In addition, the extent of acid treatment is limited. An acidified condition in the circulating water must be avoided. Thus only enough acid to reduce, but not eliminate, the alkalinity of the water can be added.
A need exists for a cost effective way to reduce scale in circulating water systems that will not have the problems associated with the prior methods.